Yep......Another prop question.

[Ingram84]

Hey guys,

I have a 1992 5.0l Cobra engine and cobra outdrive pushing a 20' ski boat. Normal for me with 4 people aboard is around 30-35 mph (per GPS) at around 3300-3500 RPM's WOT. The prop that came on the boat is a 3 blade 14 1/4 x 21 stainless steel prop. In my OMC service manual it says WOT for my setup is 4000-4400 RPM's.


My question is which way do i need to go with the prop to get 40-45 mph (or is 35 all she's got?) Also my wife loves to ski and tube. Do i lose the ability to go faster if i want a prop for skiing? And is that a good size to ski? Thanks for all the help.

 

[steelespike]

Re: Yep......Another prop question.

I'm not an expert on your setup but it seems to me your 5.0 should do better with the 21.
at least the 40 - 45 your looking for.
To gain 900 rpm you would need to go down to about 17" pitch seems extreme to me
something like what you would run for skiing with a boat load of people.
Be sure the motor is in excellent tune,do a compression check,be sure the throttle opens all the way,bottom clean and smooth,prop in excellent shape.

 

[craze1cars]

Re: Yep......Another prop question.

Bear in mind your outdrive may have been changed out to a steeper ratio at some time in this boat's nearly 20 year lifetime...if so, you will definitely need to go to a smaller prop to compensate for the steep ratio.

I agree entirely, a full tuneup and mechanical checkup is critical before an accurate prop decision can be made. After that, run it again and share your numbers. To directly answer your question, IF your boat is currently running properly, going to smaller pitch prop will not only increase your holeshot for skiing substantially, it should ALSO increase your top speed because you'll be putting the motor onto its proper powerband. Right now you are definitely off and something is wrong...either the condition/tune of your engine, or your propeller selection.

Something tells me you're maybe running on 6 or 7 cylinders. Or possibly your timing is retarded off spec. Or your throttle cable is out of adjustment. Or...?????